This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The hawkmoth, Manduca sexta, is a popular model system for studies in integrative biology. Unlike many other insects, the flight muscles are synchronous, i.e. contractions respond to individual nerve impulses similarly to vertebrate skeletal muscle. The length tension relationship of Manduca flight muscle is remarkably similar to mammalian cardiac muscle making it worth investigating as a potential model system. It has a number of other intriguing features, however. Metabolic heat production in locomotor muscles paired with surface heat loss necessarily leads to a temperature gradient. This phenomenon occurs in the dominant flight muscles involved in wing depression (dorsolongitudinal muscles: DLM) in Manduca. These muscles develop an average temperature difference of 5.6 degrees C between the warmer, ventral-most and the cooler, dorsal-most subunits. This temperature difference leads to a mechanical difference where the dorsal subunits operate with decreased contractile rates and reduced mechanical power output compared with the ventral subunits of the DLM. These results suggest that the dorsal DLMs may not be primarily involved with wing depression. Perhaps instead, the dorsal subunits, with cross-bridges remaining more attached to the thin filaments, behave as springs. Thus, at the end of the contraction they may act in concert with the antagonistic dorsoventral muscles to elevate the wings. To address this question, we conducted time-resolved small-angle x-ray fiber diffraction studies for which we monitored the movement of cross-bridges in real time. In parallel, experiments on skinned manduca flight muscle investigate basic physiological characteristics that can be compared to other muscles, including mammalian myocardium.